Digital pre-distortion (DPD) is a technique used to linearize a power amplifier in a transmitter to improve the efficiency of the power amplifier. A power amplifier in a transmitter typically must be substantially linear, so that a signal is accurately reproduced. Compression of the input signal or a non-linear relationship between the input signal and output signal causes the output signal spectrum to spill over into adjacent channels, causing interference.
A digital pre-distortion circuit inversely models the gain and phase characteristics of the power amplifier and, when combined with the amplifier, produces an overall system that is more linear and reduces distortion that would otherwise be caused by the power amplifier. An inverse distortion is introduced into the input of the amplifier, thereby reducing any non-linearity the amplifier might otherwise have.
Digital pre-distortion is typically implemented based on a Volterra series (e.g., memory polynomials or generalized memory polynomials). The complexity of these DPD algorithms increases exponentially with the memory depth of the non-linear model, and also puts constraints on how deep in time the model can be practically constructed. Performance is thus limited as this type of model only performs partial linearization. While such Volterra-based DPD techniques effectively linearize a power amplifier, they suffer from a number of limitations, which if overcome, could further reduce the complexity and improve the performance of DPD systems. A need therefore exists for improved digital pre-distortion techniques that further reduce the complexity of Volterra approximations without impairing performance.